I had this same issue happen last night to me.
I did a search. Edited the search bar again to add words to filter my search. Searched, then hit the back button and it went back to my original search, not the new edited search. I remember thinking that wasn't right. I cleared the search, closed down, restarted and tried the same search, following the same pattern. It happened again.
Normally I'm a PC/iPhone user. My iPad is my first real use with Safari. I was wondering if that was just a quirk with Safari.